Optical waveguides provide a maximum velocity of propagation for light at a single wavelength or frequency. For all other wavelengths, they provide a slightly lower velocity of propagation. This lower velocity manifests itself as a variable delay through a long optical waveguide path, where the delay depends on the light frequency or wavelength. The point at which maximum velocity is achieved depends on the design and material composition of the fiber.
Chromatic dispersion is defined as the derivative of the propagation delay with respect to the frequency of light propagated through an optical waveguide. It is to be noted that other types of dispersion exist, such as polarization dispersion for which the two polarizations (horizontal and vertical) of light are propagated at two different velocities in an optical waveguide. In the following, the term “dispersion” will refer to chromatic dispersion except where otherwise noted.
In an optical telecommunication system, the frequency dependence of the delay may be problematic. Specifically, any given optical carrier that is modulated with data contains information-related side-band frequencies differing slightly from the carrier's frequency. Since in such telecommunication systems the data is typically amplitude modulated in the optical carrier, two side-bands (an upper and a lower side-band) are present for each carrier frequency. If the carrier frequency is not close to the frequency corresponding to maximal propagation velocity, the delay experienced by the upper side-band will not be identical to the delay experienced by the lower side-band. Therefore, the upper and lower side-bands will be out of phase and can interfere destructively to reduce the amplitude of the side-band components at the receiver. Eventually, if the delay between the two side-bands is large enough, sideband components at some frequencies will be totally cancelled. Those skilled in the art will appreciate that such modifications to a signal may lead to information loss if they are not corrected. An optical signal is said to have positive dispersion polarity if the upper sideband components are delayed with respect to lower sideband components and negative dispersion polarity in the opposite situation.
Correction of dispersion in an optical transmission system is achieved by ensuring that the system taken as a whole operates over an optical path with a minimum propagation velocity differential at optical frequencies around the carrier frequency of the signal being propagated. This allows the upper and lower side-bands to experience the same propagation delays, which avoids the destructive interference described above.
In a conventional optical system, signals are propagated over multiple spans before a substantial, expensive and approximate banded dispersion compensator is deployed. In such conventional optical systems for transmitting a WDM signal comprising multiple carriers across a transmission path, WDM signals are first band-demultiplexed, which means that the incoming WDM signal is split into several other WDM signals, each having a bandwidth smaller than the bandwidth of the incoming signal and containing a lesser number of carriers. Then, each of the demultiplexed WDM signals is fed to a separate dispersion compensation system, such as those described herein below, before being fed to a receiver.
A conventional dispersion compensation system may consist of a compensating length of fiber which is added to a transmission path. The length and properties of the compensating fiber are chosen so that they have an equal but opposite effect on the dispersion of a signal propagated through an optical fiber. One disadvantage of this approach is the reduction in the width of the “low dispersion” frequency window, which comprises those frequencies that are close to the frequency for which the propagation velocity is maximal. Therefore, the “low dispersion” window includes the frequencies for which dispersion does not deform substantially a signal having a carrier frequency in this window. Those skilled in the art will appreciate that having smaller “low dispersion” window dispersion compensation devices disadvantageously leads to needing more of these devices to correct dispersion in a given signal.
Another conventional dispersion compensation system consists in having fiber transmission paths composed of a concatenated mixture of fiber types. If an appropriate choice of fiber for which dispersion characteristics compensate or partially compensate each other is used, there is no need for a separate dispersion compensator.
Those skilled in the art will appreciate that this solution is acceptable if all of the optical carriers in a given WDM stream have traversed the same optical path and have suffered substantially the same amount of dispersion. However, in a photonically switched network this is not the case. Adjacent wavelengths in an output WDM stream may have completely different ancestries and hence impairments. Therefore, a conventional approach cannot be used to correct dispersion in photonically switched networks, unless rigorous compensation is carried out on every span between switches, and even then the errors in that compensation will concatenate across the network.